Large-sized pixel image detector

ABSTRACT

A method of detecting electromagnetic radiation imparted onto a matrix of photosensitive photomos networks in which pixels of each photomos network are simultaneously exposed to electromagnetic radiation source for a predetermined period of time. Thereafter, transfer signals are applied to each photomos pixel and accumulated charges corresponding to the strength of the radiation imparted onto the pixels are transferred in a columnwise fashion to the end of each column of the photomos networks. The charges are summed at the end of the columns and placed in a reading register. The charges in the reading register of each photomos network are summed and a signal is output corresponding to the strength of the electromagnetic radiation imparted onto the photomos network.

This is a Continuation of application Ser. No 08/071,926 filed on Jun.4, 1993, now U.S. Pat. No. 5,481,301, which is aFile-Wrapper-Continuation of application Ser. No. 07/756,882, filed Sep.9, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention applies to image sensor devices formed by photosensitivesemiconductors and more particularly to such devices comprising at leastone "large-sized" photosensitive surface.

2. Description of the Related Art

The image sensor devices with semiconductors most often comprise eithermultiple photosensitive elements placed in lines and in columns, or asingle line of these photosensitive elements which then constitute astrip. Each photosensitive element corresponds to an elementary imagepoint, and the dimensions of this elementary image point are linked tothose of the photosensitive surface of the sensor element. In thedescription below, the photosensitive surface of a sensor element iscalled "pixel." It should be noted that in some cases, the image sensordevice can comprise a single photosensitive element, i.e., a singlepixel.

The pixels have dimensions which vary with the application. For example,when an effort is made to obtain a high image resolution, pixels ofsmall dimensions (for example, 10 microns×10 microns) are used. In othercases, pixels called "large sized" (for example, on the order of 100microns×100 microns) are used, in particular in an application incommunications between satellites.

The semiconductive photosensitive elements are now photodiodes or elseelements produced according to MOS (from "Mental Oxide Semiconductor")technology. In MOS technology, a capacitance is produced by depositing,on a semiconductive substrate, an insulating layer formed by an oxide,and covered by a conductive layer. This conductive layer constitutes anelectrode and it is often made of a polycrystalline silicon layer.According to this MOS technology, a sequence of such capacitances ableto form a sequence of stages of a shift register of the type with chargetransfer or, in abbreviated form, "CCD" ("Charges Coupled Device") isnow produced on the same semiconductive substrate. In these chargetransfer registers, each stage can collect charges produced by the lightand store these charges. These charges are then transferred from onestage to another to a reading register itself of the CCD type. Thus,each MOS type capacitance can constitute a photosensitive element whichis known under the name of photomos, the name under which it is calledin the description.

In the image sensors or imagers having large-sized pixels, these pixelsconsist of photodiodes. These image sensors are generally formed by Nlines comprising M photosensitive points each (with N equal to orgreater than 1 and M equal to or greater than 1). A surface imager ofthis type can be obtained by juxtaposing several lines or strips ofphotodiodes. A CCD type shift register, constituting a reading registerof which each stage corresponds to a photodiode or pixel, is associatedwith each line.

For reading, the charges of each pixel are transferred in thecorresponding stage of the reading register (simultaneously for all thelines), then the register is dumped to an output circuit making itpossible in particular to convert the charges into voltage; there aretherefore, in this case, as many outputs as there are lines.

According to another known structure, the imager is in a matrix form. Inthis case, on the one hand, each photodiode is connected to a conductorin a column by a switching element often consisting of a MOS typetransistor; on the other hand, each column is connected to a stage of aCCD type shift register. The reading is made according to a line by lineaddressing: in the "conducting" state, all of the switching elements ofthe same line are controlled so that in each column conductor, thecharges accumulated by a photodiode belonging to the addressed lineflow. These charges are stored in the corresponding stage of theregister, then the latter having been dumped, the addressing of thefollowing line is performed.

In the application to large-sized pixels, the photomos are removedbecause they present in particular as a drawback the requirement of arelatively long period to dump in a correct way the accumulated orstored charges (the step of the photomos should be compatible with thetransfer of the charges).

With the photodiodes, this defect is much less pronounced, but thephotodiodes have other serious drawbacks linked to the strongcapacitance that they exhibit. This strong capacitance of thephotodiodes in particular causes a very significant reading noise aswell as a very detrimental hangover or remanence.

SUMMARY OF THE INVENTION

This invention proposes a new architecture for an image sensor, makingit possible to produce at least one large-sized pixel, withoutencountering the drawbacks cited above. This is obtained in particularby using multiple sensors or photosensitive elements of photomos type,i.e., of CCD type, each having at a maximum a dimension compatible witha correct transfer of charges in the time assigned, to form aphotosensitive surface corresponding to a large-sized pixel, and bymaking the summation of the elementary data contained in each of thesephotomos.

According to the invention, an image sensor device comprising a networkof photosensitive surfaces placed along N lines and M columns (with Nequal to or greater than 1 and M equal to or greater than 1), eachphotosensitive surface producing charges as a function of itsillumination, is characterized in that each photosensitive surfacecomprises a network of photomos placed along j intermediate lines and iintermediate columns (with j equal to or greater than 2 and i equal toor greater than 1), each line of photosensitive surfaces comprising areading register having at least as many reading stages as there areintermediate columns, each photomos sequence along the direction of theintermediate columns constituting a shift register ending in a readingstage, each reading stage having the capacity to store all the chargesaccumulated in the corresponding intermediate column.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood better and other advantages that itobtains will come out in the reading of the following description, givenby way of nonlimiting example with reference to the accompanyingfigures, of which:

FIG. 1 is a diagram of a first embodiment of an image sensor accordingto the invention;

FIG. 2 diagrammatically shows, in more detail, the photosensitivesurface shown in FIG. 1;

FIG. 3 diagrammatically shows the photosensitive surface of FIG. 2operating with summation stages;

FIG. 4 is a timing diagram illustrating the operation of thephotosensitive surface of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 diagrammatically represents an image sensor 1 according to theinvention. Image sensor 1 comprises an image zone formed by a network ofN times M main photosensitive surfaces, each constituting a large-sizedpixel PX1 to PX75 called large pixel in the description below (a pixelis considered large-sized when these dimensions become incompatible witha correct operation in a conventional image sensor). The large pixelsare placed along N rows and M columns. In the nonlimiting example shownin FIG. 1, N=5 and M=15, and consequently, five rows L1 to L5 of largepixels PX1 to PX75 and fifteen columns C1 to C15 are represented; but inthe spirit of the invention, the number N of rows being equal to orgreater than 1, image sensor 1 can comprise a larger number of largepixels or a single large pixel PX1.

Each row L1 to L5 comprises a reading register RL1 to RL5 intended todump to an output circuit 10 the charges produced at the level of largepixels PX1 to PX75 when they are exposed to a radiation, of light, forexample.

According to a characteristic of the invention, each photosensitivesurface or large pixel PX1 to PX75 consists of a matrix network ofelementary pixels as shown in FIG. 2.

FIG. 2 is an enlarged view of a large PX15 shown in FIG. 1 and enclosedin a box formed by dashed lines, for example. FIG. 2 illustrates a largepixel PX15; with a network of j times i elementary pixels, each formedby a photomos. The photomos or elementary pixels are placed along jintermediate rows and i intermediate columns. The term "intermediate"attributed to these intermediate rows and columns makes it possible tobetter differentiate rows and columns formed by the large pixels andwhich are called "main lines L1 to L5 and main columns C1 to C15 in thedescription below. The number j is equal to or greater than 2 and numberi is equal to or greater than 1. Actually, number j defines the numberof elementary pixels in an intermediate column, and at a minimum, thisnumber should be such that the transfer of the charges is correctlyaccomplished as a function of the time assigned. However, with a singleelementary pixel per intermediate column, the invention loses itsadvantage and the minimum value for the number j is therefore 2.

In the nonlimiting example described, j=20 and i=22, as a result, 20intermediate rows LI1 to LI20 and 22 intermediate columns CI1 to CI22are represented, which form 440 elementary pixels PE1 to PE440. For thesake of convenience of representation, the dimension of elementarypixels PE1 to PE440 is shown larger in the direction of the intermediatecolumns than in the direction of the intermediate lines, but, of course,these elementary pixels can have a different geometry.

Elementary pixels PE1 to PE440 consist of photomos standard in the art,so that in the direction of intermediate columns CI1 to CI22, they canconstitute in a conventional manner a transfer stage sequence. They thuscan form a transfer register, transferring the charges in the directionof first intermediate row LI1 to a reading register RL1 located at theother end of the intermediate columns after twentieth intermediate lineLI20.

This transfer can be performed equally in biphase, three- or four-phasemode. In the nonlimiting example described, it is performed in biphasewith two transfer signals ST1, ST2 having different phases. For thispurpose, each elementary pixel PE1 to PE440 comprises, in a way standardin the art, two electrode pairs E1, E2 which follow one another in thedirection of intermediate columns CI1 to CI22 and to which are appliedtransfer signals ST1, ST2.

The reading register is a CCD shift register. It comprises at least asmany reading stages EL1 to EL22 as there are intermediate columns CI1 toCI22. Actually, reading register RL1 can be common to several largepixels, located on a same main row L1 to L5. Also, the number of readingstages should at least be equal to the total number of intermediatecolumns that a main line L1 to L5 comprises, so that in any case, areading stage corresponds to each intermediate column.

According to another characteristic of the invention, each reading stageEL1 to EL22 is designed (in its dimensions, for example) to be able tostore the sum of the amounts of accumulated charges in variouselementary pixels PE1 to PE440 which form intermediate column CI1 toCI22 to which the reading stage corresponds.

Under these conditions, the operation of image sensor device 1 is asfollows:

after a phase of exposure to the light in which all or part of thephotosensitive surface, i.e., of large pixel PX15, is exposed, transfersignals ST1, ST2 are applied to all electrodes E1, E2 of all elementarypixels PE1 to PE440. The latter then operate as registers and, in thedirection of the intermediate columns, transfer the charges optionallyaccumulated by each of them, to charge them in corresponding readingstage EL1 to EL22; reading register RL1 being stopped, of course. Eachcycle of transfer signals ST1, ST2 shifts by one position (i.e., by oneelementary pixel) to the reading register, the accumulated charges ineach of the elementary pixels. In the example illustrated in FIG. 2where each intermediate column comprises 20 elementary pixels, 20 cyclesof transfer signals are necessary for the charges produced in elementarypixels PE1 to PE22 of first intermediate line LI1 to be transferred inthe corresponding reading stage. These 20 cycles of transfer signalsST1, ST2 represent charging period TC of reading register RL; and at theend of this charging period, each stage of the reading register containsthe sum of the charges produced in entire corresponding intermediatecolumn CI1 to CI22.

At the end of this charging period TC, the reading register can dump allthe data that it contains (in the form of charges) to an output circuit(to which it is connected). The output stage can comprise, for example,in the conventional way, an amplifier 10 delivering a voltage outputsignal O.S. as a function of each amount of charges delivered by readingregister RL.

Reading register RL1 is of a standard type operating, for example, inbiphase under the action of two register transfer signals STR1, STR2having opposite phases. At the end of charging period TC, the twotransfer control signals are applied to reading register RL1, and thecharges contained in each reading stage EL1 to EL22 are successivelyapplied to output circuit 10, while the image zone integrates the nextimage.

With reference again to FIG. 1, it is possible to see in the latter thatfor each main rows L1 to L5, reading register RL1 to RL5 is common forall large pixels PX1 to PX75 belonging to the same line. Each pixel PX1to PX75 being constituted like large pixel PX15 shown in FIG. 2, thenumber of reading stages can be very large.

On the other hand, transfer signals ST1, ST2 applied to electrode pairsE1, E2 (shown in FIG. 2) are applied simultaneously to all theseelectrodes E1, E2 of all large pixels PX1 to PX75 of a same main row L1to L4 with the help of common control means (not represented).

Consequently, when the total dimension of the image sensor is large, alimitation of the operation appears, linked to the distributed timeconstants of the lines of polycrystalline silicon which form thephotomos electrodes in a standard way.

This limitation in the use of the image sensor can be very serious ifthe data flow expected at the output of output circuit 10 is high.Actually, a duration TL is used as a line period, during which, on theone hand, the charging operation in reading register RL1 to RL5 duringperiod TC, then next the reading by output stage 10 of all the datacontained in reading register RL1 to RL5 during period TC should beperformed successively: if charging period TC is increased because ofthe distributed constants, reading period t1 of M large pixels eachconsisting of i intermediate columns is reduced proportionately.

This can lead to increasing the control frequency of reading registersRL1 to RL5 to values where the operation can be very difficult toguarantee.

To minimize the control frequency of reading register RL1 to RL5, andaccording to a new characteristic of the invention, a summation stagecan be interposed between reading register RL1 to RL5 and each of theintermediate columns of all the large pixels which constitute a mainline L1 to L5. This version of the invention with summation stages issymbolized in FIG. 1 with a line LM1 to LM5 symbolized by dotted lines,and it is explained in more detail with reference to FIG. 3. It shouldfurther be noted that an output summing stage ES1 to ES5, interposedbetween each reading register RL1 to RL5 and each output circuit 10, andwhose operation is also explained with reference to FIG. 3, has beenrepresented in FIG. 1, also in dotted lines.

FIG. 3 shows fifteenth pixel PX15 by a view similar to that of FIG. 2,with the difference that, on the one hand, a summation stage EM1 to EM22is interposed between each intermediate column CI1 to CI22 and eachreading stage EL1 to EL22; and that, on the other hand, an outputsumming stage ES1 is placed in series between reading register RL1 andoutput circuit 10.

Each summation stage EM1 to EM22 is standard in the art, produced, forexample, according to the MOS technology, and it can, moreover, beconstituted as an elementary pixel PE1 to PE440.

The function of each of these summation stages is to receive almostcontinuously and to store the charges produced by all the elementarypixels of the intermediate column to which they each correspond.

Under these conditions, it is sufficient to apply transfer signals ST1,ST2 continuously to electrode pairs E1, E2 so that in each intermediatecolumn CI1 to CI22, the charges created in the intermediate column arecontinuously transferred in corresponding summation stage EM1 to EM22,while in the example described with reference to FIG. 2, the transfer inthe intermediate columns is limited to charging period TC. At the end ofline period TL, i.e., at the end of the integration period or period ofexposure to the light, the charges accumulated in each summation stageEM1 to EM22 are transferred almost instantaneously in correspondingreading stage EL1 to EL22. Actually, it is sufficient for this purposefor a transfer step, which is made under the control of a memorytransfer signal STM applied simultaneously to all summation stages EM1to EM22.

Transfer control signals STR1, STR2 are then applied to reading registerRL1, to dump to output circuit 10 the data or charges contained in eachof reading stages EL1 to EL22.

This method leads to reducing, to the point of virtually eliminating,charging period TC, and thus makes it possible to give virtually theentire duration of line period TL to the transfers in reading registersRL1 to RL5, i.e., to the reading of data or charges by output circuit10.

Thus, the frequency of the control signals applied to reading registersRL1 to RL5, and the frequency of transfer signals ST1, ST2 applied toelectrode pairs E1, E2, i.e., to the image zone, are minimized at thesame time. Actually, the transfer in the intermediate columns beingcontinuous, its duration for an entire intermediate column, i.e., theduration of the transfer of j intermediate lines, can be brought to themaximum which is the duration of a line period TL.

According to another characteristic of the invention, an output summingstage ES1 to ES5 is interposed between reading register RL1 and outputstage 10. This summing stage makes it possible to sum the amounts ofcharges contained in a given sequence of reading stage of readingregisters RL1 to RL5 and corresponding to i intermediate columns (i=22in the example) of a large pixel; the charges can be delivered bysumming stage ES1 in the form of bundles, each comprising the sum of thecharges delivered by all the intermediate columns of a large pixel. As aresult, during the period of line period TL, the flow of output fromoutput circuit 10 corresponds to the reading of M large pixels, and nolonger of M times i intermediate columns. As a result, the passbandnecessary for the amplifier of output circuit 10 can be reduced, whichleads to a significant reduction of the "RMS" ("Root Mean Square")output reading noise of the image sensor. Summing stage ES1 of output 20can be constituted, for example, as a summation stage EM1 to EM22. It issufficient to control the transfer of the charges that it contains, tooutput circuit 10, with a transfer control signal SCT which is appliedto it with a phase relation determined relative to the transfer of thereading register; a transfer control signal SCT being applied to summingstage ES1 because of i transfers in reading register RL1. Of course,summing stage ES1 should have the capacity to store all the chargesproduced by a large pixel PX1 to PX75, i.e., the sum of the chargesproduced by j times i elementary pixels.

Output summing stage ES1 can be interposed between the reading registerand output stage 10, both in the version with summation stages EM1 toEM22 and in that described with reference to FIG. 2.

FIG. 4 is a timing diagram which illustrates, by lines A to G, by way ofnonlimiting example, the operation of the image sensor in the versionwith summation stages EM1 to EM22 and output summing stage ES1 to ES5.

Line A represents the duration of a line period TL, period which isdefined between two successive transfer signals STM applied to summationstages EM1 to EM22.

Lines B and C respectively represent a sequence of first and secondtransfer signals ST1, ST2 applied to electrode pairs E1, E2 of eachelementary pixel PE1 to PE440, with opposite phases. Taking into accountthe described example in which each intermediate column comprises 20elementary pixels, 20 transfer signals ST1, ST2 follow one another inthe period of a line period TL.

Lines D and E represent two register transfer signals STR1, STR2 appliedto reading registers RL1 to RL5. By assuming that main lines L1 to L5 oflarge pixels PX1 to PX75 contain 15 large pixels each and that eachlarge pixel contains 22 intermediate columns, it is necessary thatduring the period of line period TL, each reading register RL1 to RL5produces 15 times 22 transfers to output summing stage ES1 to ES5; thelatter under the control of register transfer signals STR1, STR2. Toperform this operation, signals STR1, STR2 have been grouped in bundlesof 22 signals each.

Line F represents a sequence of transfer control signals SCT applied tooutput summing stage ES1 to ES5, during the period of line period TL, ata rate of one control signal SCT for 22 register transfer signals STR1,or STR2, i.e., for each bundle of these signals as they are representedin lines D and E. Each of these signals has the effect of applying tooutput circuit 10 an amount of charge which is the total amount producedby a large pixel.

Line G illustrates the sequence of output signals O.S. delivered byoutput circuit 10, at the same frequency as transfer control signals SCTapplied to output summing stage ES1 to ES5.

What is claimed is:
 1. A method for detecting electromagnetic radiationimparted onto a network of photosensitive surfaces, each photosensitivesurface comprising a photomos network of at least one column of at leasttwo lines of elementary pixels, said at least one column forming avertical shift register having P photosensitive pixels, including oneend pixel, and said sensor comprising a horizontal reading registerhaving one reading stage at one end of each column, said methodcomprising the steps of:simultaneously exposing all of the photomosnetworks of at least one photosensitive surface to electromagneticradiation during a predetermined period of time, thereby chargingelementary pixels of each photomos network; applying, after thepredetermined period of time has ended, transfer signals to eachvertical shift register so as to shift in P steps, the charges of allthe elementary pixels of the corresponding column towards said one endof the column and shift correspondingly P times, the charges of the endpixel of the column into the reading stage of the corresponding column;summing in the reading stage of the corresponding column the chargesreceived from the end pixel of the column during the P shift steps, saidcharges resulting from exposure of all the pixels of the column toelectromagnetic radiation during the said predetermined period of time;and applying transfer signals to said horizontal reading register so asto transfer towards an output of the horizontal reading register thecharges contained in the reading stages.
 2. A method for detectingelectromagnetic radiation imparted onto a network of photosensitivesurfaces, each photosensitive surface comprising a photomos network ofat least one column of at least two lines of elementary pixels, said atleast one column forming a vertical shift register having one end pixel,and said sensor comprising a horizontal reading register having onereading stage at one end of each column, said method comprising thesteps of:simultaneously exposing all of the photomos networks of atleast one photosensitive surface to electromagnetic radiation forsuccessive predetermined periods of time, thereby charging elementarypixels of each photomos network; applying, after each predeterminedperiod of time, transfer signals to the shift register of each column soas to shift one step towards said one end of the column the chargesaccumulated in the elementary pixels of the column and to transfer fromthe end pixel of the column into a summation stage, connected betweensaid one end of the column and the corresponding reading stage, thecharges contained in said end pixel; summing in said summation stage thecharges transferred from said end pixel during K successivepredetermined periods of time; periodically transferring, after Ksuccessive periods of time, the charges thus accumulated in saidsummation stage, into the corresponding reading stage of the readingregister; and applying transfer signals to the reading register so as toshift towards an output of the reading register the charges contained inthe reading stages.
 3. An image sensor comprising:a network ofphotosensitive surfaces, each photosensitive surface comprising aphotomos network of at least one column of at least two lines ofelementary photosensitive pixels, said at least one column forming aphotosensitive vertical shift register having one end pixel, and saidphotomos networks being simultaneously exposed to electromagneticradiation for successive predetermined periods of time; a horizontalreading register for receiving charges from the network and transferringthe charges towards an output of the sensor, said register comprising arespective reading stage for each column; a respective summation stageconnected to one end of each column, between said end pixel of thecolumn and a corresponding reading stage of the horizontal register, forsumming the charges received from said end pixel during K successiveperiods of time; means for applying, after each predetermined period oftime, transfer signals to the vertical shift register of said at leastone column so as to shift one step towards said one end of thecorresponding column the charges accumulated in the elementary pixels ofthe column during said predetermined period of time, and so as totransfer to the corresponding summation stage the charges contained insaid end pixel of the column; means for periodically transferring in thereading stage, after a group of K successive periods of time, thecharges accumulated in the summation stage during, said K successiveperiods of time; and means for transferring towards an output of thereading register the charges transferred in the reading stages at theend of the group of K successive periods.